The present invention relates to machines for spraying slurries of settable substances such as plaster or gypsum cement, and more specifically, to an improved spray machine suitable for spraying the slurry upon a vertical surface so that the slurry sets quickly.
In industrial, architectural and tooling applications, such as the fabrication of molds for the construction of large fiberglass or plastic items such as boat hulls, it has been contemplated to create a master form out of a sprayable plaster, also known as calcium sulfate hemihydrate composition. Unlike prior sprayable plaster slurries, the present slurry composition may have relatively high viscosity and/or high cohesiveness, and is preferably formulated to be readily machinable so that once sprayed and set upon the substrate, the form can be shaped using conventional machining tools.
However, in developing a suitable spray apparatus for achieving this goal, it has been found that conventional systems for spraying plaster are unsuitable for this application. Conventional plaster slurry spray machines utilize spray gun designs such as pole guns which are unwieldy and lack the necessary precision, are designed for spraying paint or resin and are typically internal mix designs.
More specifically, conventional spray machines employ simple peristaltic or progressive cavity pumps fed by a drum or other type of slurry storage/mixing container. The slurry is pumped and delivered to an atomizing device ("spray gun") which range from an internally atomized to an externally atomized device which breaks up the slurry with compressed air. It is known in such devices to incorporate so-called chopper guns, which are fed a supply of rope-like fiberglass roving, and chop the roving into fibers which are then incorporated into the slurry to add strength.
It is also known in such devices to inject a stream of accelerant (also referred to herein as "accelerator") into the slurry to reduce the set time once the slurry is sprayed on the substrate. If the accelerant is injected internally (within the gun), it has been found that the slurry begins to prematurely set inside the gun, causing clogging and interfering with cleaning of the spray apparatus. Such systems require frequent maintenance to remove any buildup of set particles. Externally supplied accelerants have been found to supply a non-uniform distribution of accelerant to the slurry stream, causing spotty plaster set times. In such cases, the resulting product has an uneven appearance and cannot be worked as rapidly as desired.
A design criterion of such systems is that peristaltic pumps are typically used. However, a drawback of such a system is that the slurry is delivered in a pulsating flow which is not uniform. This causes difficulties for both the operator and the uniform integration of accelerant or chopped fiberglass into the plaster slurry stream. Progressive cavity pumps deliver slurry at a constant rate, but are expensive and difficult to clean and maintain.
Further, conventional slurry spray systems are equipped with ball valves, which are prone to plaster build-up and premature failure. Lastly, conventional slurry spray systems do not provide the user with the ability to finely control slurry output.
Accordingly, there is a need for a spray apparatus configured for spraying a settable slurry such as a plaster slurry, which is relatively low maintenance, hinders clogging caused by premature setting of the slurry, yet provides for the incorporation of an accelerant into the slurry, which provides for a relatively uniform or constant output of slurry and which provides for a relatively finite output adjustability controlled by the user.
Thus, it is a first object of the present invention to provide an improved slurry spray apparatus which incorporates an accelerant into the slurry without internal clogging, and so that the accelerant is generally uniformly distributed in the slurry.
Another object of the present invention is to provide an improved slurry spray apparatus which emits or ejects the slurry at a relatively constant rate to achieve a more uniform sprayed substrate.
A further object of the present invention is to provide an improved slurry spray apparatus in which the component parts such as pumps and valves are designed to accommodate the unique characteristics of plaster slurries for efficient industrial applications.
A still further object of the present invention is to provide an improved slurry spray apparatus which features a pneumatically operated control and valve system for operator safety and for more controllable output of the sprayed slurry.